Yttria-stabilized zirconia membrane electrode

ABSTRACT

The present invention is an improved yttria-stabilized zirconia electrode having a ceramic tube ( 12 ). The electrode is improved by replacing the method of sealing the electrode with an epoxy seal ( 24 ) and filling the tube with a ceramic glue ( 42 ) without completely filling the tube. The ceramic glue is added in a small amount and the ceramic glue is heated in the tube before a next small amount of ceramic glue is added, until the final amount of ceramic glue is added and heated. Also, an area of cover on a wire ( 18 ) in an area between a top of the ceramic glue and a top of the tube is partially removed. A sealing glue above the top of the ceramic glue that adheres to the wire at the removed area of the cover, adheres to the tube and seals the top of the tube. Finally, an outside portion of the wire which extends beyond the top of the tube is gripped with a CONAX fitting ( 26 ), instead of the fitting gripping the tube.

This application claims priority to U.S. Provisional Application No.60/109,977 filed Nov. 25, 1998, which is herein incorporated byreference.

BACKGROUND

High temperature and high pressure aqueous solutions have an enormousnumber of applications in sciences and industries, includinggeosciences, synthesis and deposition of ceramics, supercritical wateroxidation, fossil, nuclear, and geothermal power generation, pulp andpaper production, and chemical productions. Potentiometric and pHmeasurements provide important information for studying thermodynamicsand electrochemistry in high temperature aqueous solutions and in-situmonitoring and control of physical and chemical processes, e.g.speciation and corrosion, in industrial systems. More and moreelectrochemical sensors are used in industries. However, there are onlya few devices having pH sensing electrodes for use during measurement inhigh temperature environments. Currently, the only primary and reliablepH sensing electrode available for use at high subcritical andsupercritical temperatures (>374° C.) is the Yttria-Stabilized Zirconia(YSZ) membrane electrode.

Two typical designs of YSZ electrodes used in the laboratory are shownin FIGS. 1 and 2. The electrode 10 of FIG. 1 is a YSZ tube 12 with aclosed tip 14 and an open top 22. The closed tip 14 of the tube 12 isfilled with a small amount of Hg|HgO paste 16. A Platinum (Pt) wire 18that is partially covered with a shrinkable PTFE tube is placed in thetube 12, whereby the uncovered portion of the PT wire 18 is insertedinto the Hg|HgO paste 16. The tube 12 is then filled with zirconia sand20. The Pt wire 18 is used to provide electrical contact. The open top22 of the tube 12 is typically sealed with epoxy 24 over the sand 20.The whole tube 12 is fitted into a CONAX fitting 26 at the top 22 of thetube 12 whereby the CONAX fitting 26 is a tube fitting used for grippingand sealing a tube. The wall of the tube 12 is the boundary separatingthe high pressure fluid (typically 300 bars) and the low pressureoutside of the system. Under high pressures, the tube 12 will break,especially at the point where the CONAX fitting 26 grips t he YSZ tube12. The electrode 27 of FIG. 2 is designed to eliminate breaking at thegripping location of the CONAX fitting 26. The electrode 27 utilizescomplex techniques of joining ceramics to metals, such as brazing, toproduce a tube 12 having an upper portion 28 of metal and a lowerportion 30 of ceramic. Thus, allowing the CONAX fitting 26 to grip themetal upper portion 28 part of the tube 12. The problem with themetal-ceramic electrode 27 of FIG. 2 is that the metal-ceramic joint isnormally not strong enough to sustain high temperatures and highpressures for a long period of time. Other methods of joining metals toceramics have been tried or proposed. However, these methods are eithernot reliable or extremely costly.

The object of the present invention is to provide a more reliable YSZmembrane electrode.

SUMMARY OF THE INVENTION

The present invention is an improved yttria-stabilized zirconiaelectrode having a ceramic tube. The electrode is improved by replacingthe method of sealing the electrode with an epoxy seal and filling thetube with a ceramic glue without completely filling the tube. Theceramic glue is added in a small amount and the ceramic glue is heatedin the tube before the next small amount of ceramic glue is added, untilthe final amount of ceramic glue is added and heated. Also, an area ofcover on a wire in an area between a top of the ceramic glue and a topof the tube is partially removed. A sealing glue above the top of theceramic glue that adheres to the wire at the removed area of the cover,adheres to the tube and seals the top of the tube. Finally, an outsideportion of the wire which extends beyond the top of the tube is grippedwith a CONAX fitting, instead of the fitting gripping the tube.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a prior art design;

FIG. 2 is a cross-sectional view of another prior art design;

FIG. 3 is a cross-sectional view of an electrode according to thepresent invention;

FIG. 4 is schematic of a thermocell for high temperature tests using thepresent invention;

FIG. 5 is test data demonstrating the accuracy of the present invention;and

FIG. 6 is test data demonstrating the accuracy of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is an improved YSZ pH sensing electrode 40, asshown in FIG. 3. There are two differences which overcome thedeficiencies of the current electrodes available. First, on top of thezirconia sand 20, a ceramic glue 42 is instead of the epoxy 24. Whereby,the sand 20 is not filled to its usually height to allow for moreceramic glue 42. The ceramic glue 42 should be filled in the tube 12using the following the steps: 1) add one centimeter of glue 42; 2)preheat the tube at 120° C. for 24 hours; 3) heat the tube at 600° C.for 5 hours; 4) add another centimeter of glue 42; 5) preheat the tubeat 120° C. for 24 hours; and 6) heat the tube at 600° C. for 5 hours.The ceramic glue 42 should only partially fill the tube 12 as shown inFIG. 3 to allow room for Chembond glue 44 that is also shown. TheChembond glue 44 is a glue which adheres to polymers and ceramics.Following the above-described procedure allows the glue 42 to stronglyadhere to the inside wall of the YSZ tube 12. Second, the PTFE cover 46on the Pt wire 18 is etched with active sodium in the area between theof the ceramic glue 42 and the top 22 of the tube 12 to remove some ofthe PTFE cover 46 on the Pt wire 18. This area of the tube 12 is thenfilled with the Chembond glue 44 to seal the tube 12. The twoabove-mentioned procedures allow a tight bond to form between the tube12 and the ceramic glue 42 and between the Pt wire 18 and the Chembondglue 44 and tube 12. The CONAX fitting 26 is fitted to grip PTFE cover46 of the Pt wire 18 which extends from the top 22 of the tube 12instead of the tube 12 itself, in contrast to the current availableelectrodes. The whole ceramic tube 12 now ready for use in high pressurefluid.

FIG. 4 presents a schematic of a thermocell for high temperature testsin which YSZ pH sensing electrodes were tested together with aflow-through external pressure-balanced reference electrode (FTEPBRE)and a flow-through hydrogen (Pt) electrode (FTHE) at temperatures up to350° C. and a pressure of 248 bars. It is known that if the dissolvedhydrogen concentration is a constant, both FTHE and YSZ electrode shouldrespond identically to the pH of test solutions. The test data is givenin FIGS. 5 and 6 demonstrate that YSZ operates correctly under the testconditions for a long period of time, noting that the potentialdifferences for two solutions are almost the same for the YSZ electrodeand the hydrogen electrode.

While different embodiments of the invention has been described indetail herein, it will be appreciated by those skilled in the art thatvarious modifications and alternatives to the embodiments could bedeveloped in light of the overall teachings of the disclosure.Accordingly, the particular arrangements are illustrative only and arenot limiting as to the scope of the invention which is to be given thefull breadth of any and all equivalents thereof.

We claim:
 1. A method of making a yttria-stabilized zirconia electrodehaving a ceramic tube, the improvement comprising: a. replacing themethod of sealing the electrode with an epoxy seal and filling the tubewith a ceramic glue without completely filling the tube; b. removingpartially an area of cover on a wire in an area between a top of theceramic glue and a top of the tube; c. adding a sealing glue above thetop of the ceramic glue that adheres to the wire at the removed area ofthe cover, adheres to the tube and seals the top of the tube.
 2. Themethod of claim 1, further including gripping an outside portion of thewire which extends beyond the top of the tube with a tube fitting usedfor gripping the tube.
 3. The method of claim 1, wherein said ceramicglue is added in a small amount and the ceramic glue is heated in thetube before a next small amount of ceramic glue is added, until thefinal amount of ceramic glue is added and the tube is again heated. 4.The method of claim 3, wherein said ceramic glue is filled and heated inthe tube using the following the steps: 1) add one centimeter of cement;2) preheat the tube at 120° C. for 24 hours; 3) heat the tube at 600° C.for 5 hours; 4) add another centimeter of cement; 5) preheat the tube at120° C. for 24 hours; and 6) heat the tube at 600° C. for 5 hours. 5.The method of claim 1, wherein the sealing glue is a glue which adheresto polymers and ceramics.
 6. The method of claim 1, wherein the cover onthe wire is removed using active sodium applied in the area to beremoved.
 7. A yttria-stabilized zirconia electrode comprising: a ceramictube having a bottom with a closed tip and an open top; a wire having acover that is inserted into said open top, whereby an end of said wireis exposed, said end being positioned in said closed tip and whereby aportion along said wire is exposed in an area within said tube and nearsaid open top; sand which partially fills said tube; a ceramic gluefilled above said sand, whereby said ceramic glue is inserted and heatedin said tube in increments to a height in said tube which is below saidportion of said wire which is exposed; a sealing glue filling the tubeabove said ceramic glue and adhering to said portion of said wire whichis exposed.
 8. The yttria-stabilized zirconia electrode of claim 7,further including a tube fitting griping said covered wire extendingfrom said open top of said tube.
 9. The yttria-stabilized zirconiaelectrode of claim 7, wherein said ceramic glue is filled and heated inthe tube using the following the steps: 1) add one centimeter of cement;2) preheat the tube at 120° C. for 24 hours; 3) heat the tube at 600° C.for 5 hours; 4) add another centimeter of cement; 5) preheat the tube at120° C. for 24 hours; and 6) heat the tube at 600° C. for 5 hours. 10.The yttria-stabilized zirconia electrode of claim 7, wherein said sandis zirconia.
 11. The yttria-stabilized zirconia electrode of claim 7,wherein said cover of said wire is a shrinkable polytetrafluoroethylenetube.
 12. The yttria-stabilized zirconia electrode of claim 7, whereinsaid closed tip of said tube is filled Hg|HgO paste.
 13. Theyttria-stabilized zirconia electrode of claim 7, wherein said wire is aplatinum wire.
 14. The yttria-stabilized zirconia electrode of claim 7,wherein said sealing glue is a glue which adheres to polymers andceramics.
 15. The method of making a yttria-stabilized zirconiaelectrode comprising: a. inserting a wire into an open top of a ceramictube having a closed tip, said wire having a cover, whereby an end ofsaid wire is exposed, said end being positioned in said closed tip andwhereby a portion along said wire is exposed in an area within said tubeand near said open top; b. filling said tube partially with sand; c.filling said tube above said sand with a ceramic glue, whereby saidceramic glue is inserted and heated in said tube in increments to aheight in said tube which is below said portion of said wire which isexposed; d. filling said tube with a sealing glue above said ceramicglue, whereby said sealing glue adheres to said portion of said wirewhich is exposed and adheres to said tube.
 16. The method of claim 15,further including griping said wire extending from said open top of saidtube with a connector means which grips and seals said tube.
 17. Themethod of claim 15, wherein said sand is zirconia.
 18. The method ofclaim
 15. wherein said cover of said wire is a shrinkablepolytetrafluoroethylene tube.
 19. The method of claim 15, wherein saidclosed tip of said tube is filled Hg|HgO paste.
 20. The method of claim15, wherein said wire is a Platinum wire.
 21. The method of claim 15,wherein said sealing glue is a glue which adheres to polymers andceramics.
 22. The method of claim 15, wherein said ceramic glue isfilled and heated in the tube using the following the steps: 1) add onecentimeter of cement; 2) preheat the tube at 120° C. for 24 hours; 3)heat the tube at 600° C. for 5 hours; 4) add another centimeter ofcement; 5) preheat the tube at 120° C. for 24 hours; and 6) heat thetube at 600° C. for 5 hours.